Human cytomegalovirus (HCMV) is the leading infectious cause of congenital birth defects in the United States, and is an important source of morbidity and mortality for those who are immunocompromised. Since a substantial proportion of HCMV transmission occurs perinatally and in early infancy, an ideal HCMV vaccine would be effective in this age group. A potential limitation of vaccination at this age is the apparent immaturity of CD4 T cell expression of effector molecular, particularly secreted cytokines, e.g., interferon-gamma (IFN-gamma) and tumor necrosis factor (TNF), surface molecules, such as CD40-ligand (CD40-L). Such limitations have not been shown for postnatally acquired HCMV infection. The first aim of this project is to determine HCMV-specific CD4 T cell expression of effector molecules, particularly secreted cytokines, e.g., interferon-gamma (IFN-gamma) and tumor necrosis factor (TNF), surface molecules, such as CD40-ligand (CD40-L). Such limitations have not been shown for postnatally acquired HCMV infection. The first aim of this project is to determine HCMV-specific CD4 T cell memory/effector function following primary HCMV infection of infants and adults, and its relationship with other HCMV-specific immune responses, viral clearance, and the establishment of viral latency. It is hypothesized that young infants will have a decreased accumulation of HCMV antigen-specific memory/effector CD4 T cells that can secrete cytokines or express CD40-L compared to adults. We further predict that infant HCMV-specific CD4 T cells will preferentially produce Th2 cytokines that lack anti-viral activity or many inhibit this activity. Responses to HCMV overall, and to the pp65 matrix protein, which appears to be a major target of the CD4 T cell response, will be examined. It will be determined if decreased and/or altered infant CD4 T cell responses are paralleled by decreased serum levels of neutralizing HCMV antibody, prolonged viremia, prolonged urinary viral shedding (all determined by the Clinical Research Core), decreased HCMV-specific CD8 T cell function (Project 2), and decreased antigen-presenting function (APC) (Project 3). The second aim is to develop fluorochrome- labeled HLA-DR tetramer molecules containing ppo65 peptides to identify CD4 T cells in infants and adults with specificity for pp65. This will test the hypothesis that infants but not adults have a substantial fraction of CD4 T cells with specificity for pp65 but reduced or absent responsiveness to pp65 antigen. These studies will provide insight into the human CD4 memory/effector T cell response following primary HCMV infection in normal infants and adults, APC function. Since new trials of HCMV vaccines are imminent, these studies may also provide useful immature response predictors for vaccine efficacy, such as prevention of HCMV infection and/or improved maintenance of latency. They may also point out strategies by which protective responses might be augmented by immunotherapy, particularly during infancy.